Planet Earth
Igneous Rocks (2) -- Igneous StructureGeologists are interested in describing features that they see in the rocks and in trying to group them according to shape, size, rock type, etc. In the broadest sense a structure is any feature that can be seen in the rock or in a sequence of rocks. It might be a sedimentary layer, a dinosaur footprint, a fold in a metamorphic rock, or a great body of granitic rocks. The reason for being interested in structures is that by describing them, one can begin to ask and answer the question of how they formed. If they are common, then they must be formed by a process that happens fairly often. If the structure is only seen in igneous rocks, then if must related to the way igneous rocks form. In this lecture, we will examine some of the common structures and propose possible explanations for how they form.
I. Structure -- definition and use
A. Map and cross section
B. Seeing the Earth's interior in its surface
C. Relating structure to geologic process
II. Structures related to intrusion
A. Principles of isostacy
B. Dikes and sills
C. Plutons ('fossil' magma chambers)
III. Structures related to volcanic activity
B. Joints and vesicles
C. Flows
D. Pillows
Reading: Chapter 6 pp 113 -- 117
Objectives:
1. Recognize map and cross section views of geologic features.
2. Discuss the value of understanding structure.
3. Relate dikes and sills to the principle of isostacy.
4. Explain the geologic significance of batholiths.
5. Explain why lava takes on the characteristics of pahoehoe and aa lava.
6. Explain why joints form in lava flows.
7. Relate the appearance of a particular volcano to the type of magma that erupts from it.
8. Pillow lavas are found along I-95 in Maryland. Explain the geologic significance of this find.